1. Field of the Invention
The present invention relates to an intake system for allowing air to be taken into an engine and a motorcycle including such a system.
2. Description of the Related Art
Various kinds of variable valve mechanisms that control intake/exhaust have been developed in order to improve fuel consumption, reduce harmful substances in exhaust gas, and achieve high power output in a target engine speed range (see, for example, JP 2003-314233 A).
A valve device disclosed in JP 2003-314233 A includes a cam having a main cam surface and a valve lifter pressed by the main cam surface to move the valve.
The above-described cam rotates together with the camshaft and is movable in the axial direction so that the cam moves in response to the engine speed. Therefore, the contact point between the cam and the valve lifter axially moves on the main cam surface described above depending on the engine speed. Here, the main cam surface described above is inclined along the axial direction. Therefore, in the valve device, the valve lift amount is continuously changed in response to the engine speed and the accelerator position.
More specifically, a valve device disclosed by JP 2003-314233 A allows an intake air amount to be continuously adjusted in response to the engine speed and the accelerator position. The engine can smoothly be operated in this way.
With the structure of the valve device disclosed by JP 2003-314233 A, however, the amount of air taken into cylinders cannot be adjusted appropriately when there is a malfunction in the valve device. Therefore, the output of the engine cannot be adjusted sufficiently and the traveling performance of the motorcycle is lowered.